(1) Field of the Invention
The present invention relates to a liquid crystal display for displaying images through a liquid crystal display panel, in particular, relating to a liquid crystal display which is improved in the optical response characteristics upon occurrence of a gray scale transition in the liquid crystal display panel.
(2) Description of the Prior Art
Recently, personal computers, television receivers and the like use flat panel type displays such as liquid crystal displays (LCDs). A liquid crystal display includes a liquid crystal display panel made up of two glass substrates with electrodes, a liquid crystal substance injected between them and a driver circuit, and controls the amount of transmitted light by changing the strength of the electric field between the electrodes so as to display a desired image.
In order to increase the response speed, most liquid crystal display panels utilize TFT LCDs which employ a thin-film transistor (TFT) switching element for each pixel.
Recently, liquid crystal displays for computers, to say nothing of those used for television receivers, have become required to display motion pictures. Therefore, there are response speed deficiency problems even with TFT LCDs. To deal with this, a technique has been devised, in which high-level signals or low-level signals are temporarily applied when drive signals for the liquid crystal display panel are changed. This method, however, tends to produce overshoot when the voltage changes from low to high levels and undershoot when the voltage changes from high to low levels, resultantly being unpreferable in view of visibility.
As countermeasures against this problem, a technique is published in Japanese Patent Application Laid-open 2002-62850, in which signal correction is performed based on a compensation signal table registered in table memory so as to compensate the optical response characteristics of the LCD panel.
FIG. 1 is a block diagram showing a conventional liquid crystal display, and FIG. 2 is an illustrative view showing a signal table.
This liquid crystal display includes a LCD panel 10, a reference table memory 11, a frame memory 12, a control circuit 13, a data input terminal 14, a synchronizing signal input terminal 15, a data bus 16 for reference table memory 11 and an address bus 17 for reference table memory 11.
In this liquid crystal display, it is assumed that LCD panel 10 handles 8 bit display data, data with 256 levels of gray scales. The image data input from input terminal 14 is given to the 8 bits of the address of reference table memory 11 as well as to frame memory 12. The frame memory 12 outputs image data, one display period before, or delayed by one display period, and this delayed data is input to the remaining 8 bits of the address of reference table memory 11.
In reference table memory 11, compensation signal data (actual measurement) for enabling LCD panel 10 to achieve proper optical response within one display period when a signal level transition takes place is written in beforehand for all the gray scale transitions, or for all the combinations of variation of signal levels. For example, as shown in FIG. 2, the compensation signal data is represented in a 256×256 matrix form, so that a combination of the image data values of the current vertical display period and the previous vertical display period will determine a compensation signal data value to be written into LCD panel 10.
By preparing the reference table memory with the above compensation signal data written in, the desired display signal level, continuously determined based on the signal level to be displayed at present and the signal level at the time one display period ago, can be applied as the compensation signal data to LCD panel 10, whereby it is possible to achieve high-speed response display with compensated optical response characteristics of LCD panel 10 for any possible signal level change (gray scale transition).
When the compensation signal data values as to all the gray scale transitions of 256×256 patterns, from one to the next vertical display periods, are stored in the memory as in the reference table in FIG. 2, a large memory capacity is needed. Therefore, as shown in FIG. 3, nine representative gray scale levels may be extracted at uniform intervals of every 32 levels of gray scales, for example, from the 256 levels in each vertical display, of one vertical display and of the next, so as to create a table for storing compensation signal data for only the 9×9 gray scale transition patterns while the compensation signal data corresponding to the gray scale transition patterns for gray scale levels other than the above representative gray scale levels may be determined by linear interpolation using the compensation signal data in the above reference table.
In the above way, when compensation signal data (actual measurement) for the representative gray scale levels only is stored in the reference table memory, it is possible to reduce the amount of memory compared to the reference table memory for storing the compensation signal data (actual measurement) for all the transitions of 256×256 patterns. However, since the compensation signal data for the gray scale levels between the representative gray scale levels should be determined by linear interpolation, the precision of the compensation signal data cannot help but degrade.
To sum up, there exists a tradeoff relationship between the amount of memory for the reference table memory and the compensation accuracy of the optical response characteristics of the LCD panel, hence it has been difficult to improve the compensation accuracy of the optical response characteristics of the LCD panel while reducing the amount of memory for the reference table memory.